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early 15c., "mythical, legendary," from Latin fabulosus "celebrated in fable; rich in myths," from fabula (see fable (n.)).
Sense of "incredible" first recorded c.1600. Slang shortening fab first recorded 1957; popularized in reference to The Beatles, c.1963.
Fabulous (often contracted to fab(s)) and fantastic are also in that long list of words which boys and girls use for a time to express high commendation and then get tired of, such as, to go no farther back than the present century, topping, spiffing, ripping, wizard, super, posh, smashing. [Gower's 1965 revision of Fowler's "Modern English Usage"]Related: Fabulously.